Such ram boring machines in which the automatic striking piston applies periodic ramming blows on a moveable impact tip or on the machine housing are known, for example from German Patent Specifications Nos. 21 57 259 and 26 34 066. The ram boring machines serve primarily to lay service lines such as, for example, gas and water lines, electricity or telephone lines or cables under roadways or sidewalks without the need to tear up the surface of the roadway or the sidewalk at the same time. As the ram boring machine moves through the ground it forces the soil aside and leaves behind it a passage into which a service line can be simultaneously or subsequently drawn. Such a ram boring machine can therefore be connected at its rear end to a coupling for fastening a follower pipe, or alternatively a pipe to be laid can be pulled into the passage in the ground by the ram boring machine using a tow rope.
It is known from German Patent Specification No. 28 24 915 to use such ram boring machines for the destructive replacement of old lines, for example comprising iron or ceramic pipes. For this purpose the impact tip of the machine is provided with cutting edges or impact knives that project radially outwards. As the impact tip of the ram boring machine moves forwards the cutting edges burst the old pipe line. In this case too a follower pipe coupled to the housing or held by a tow rope can be pulled simultaneously in one operation into the ground. Common to all the numerous ram boring machines known is that they are supplied by a compressed air hose connected to the rear end of the housing and having its other end connected to a compressed air source, which may be very remote. This leads to problems particularly if the piping used is in the form of many short pipes put together, as is particularly common in drainage engineering when laying or replacing drain pipes, and not of long continuously welded pipe strings as, for example, in the case of laying or replacing gas pipes.
Such short pipes, which are often not more than a meter long, can either be pushed into the passage in the ground with a pressure driving unit or--as disclosed in German Patent No. 26 11 677--can be pulled in behind a steel cable. The compressed air hose, which then runs through the pipe to be pulled in and is coupled to the rear end of the machine housing, is however extremely disadvantageous and requires troublesome additional measures to be taken because assembled pipes of steel and plastics material are welded at their joints. To avoid damage by the heat generated when welding the compressed air hose must not be in the pipe being drawn in, and must be uncoupled and pulled out of the pipe before each welding operation. Plastics material pipes are as a rule heated with the aid of a thermal shield inserted into the pipe, which must therefore have a through bore for the compressed air hose, and are then pressed together. The individual pipe strings or short pipe must--in order to avoid uncoupling the hose each time--be threaded on to the compressed air hose. A pressure driving press must have a bore allowing the compressed air hose to be passed through it. The hose must also be uncoupled in order to be able to put the pressure driving unit into position.